Retcon retcon retcon. We see it a lot. Years erased, friends and family written out, romances that never were.
I think it would be interesting if the characters figured it out. Batman is supposed to be the worlds greatest detective, right?
So have him figure out what has happened. Maybe recover the memories of a bat or two from a previous cycle. Hell, maybe for laughs sandbag him with adam west batman. Hard to crunch numbers when you're dancing the batusi.
The threat too, holy poo. No cataclysm, no crisis, no villain to defeat, no meteor to deflect.
Maybe it would give the ends a bit more dignity. They realize it's the end, and we get to see how they cope with it.